sh
—
command interpreter (shell)
sh |
[ -/+abCEefhIimnPpTuVvx ]
[-/+o
longname ]
[] |
sh |
[ -/+abCEefhIimnPpTuVvx ]
[-/+o
longname ]
-c
string
[] |
sh |
[ -/+abCEefhIimnPpTuVvx ]
[-/+o
longname ]
-s
[arg ... ] |
The
sh
utility is the standard command
interpreter for the system. The current version of
sh
is close to the
IEEE Std 1003.1 (“POSIX.1”)
specification for the shell. It only supports features designated by POSIX,
plus a few Berkeley extensions. This man page is not intended to be a tutorial
nor a complete specification of the shell.
The shell is a command that reads lines from either a file or the terminal,
interprets them, and generally executes other commands. It is the program that
is started when a user logs into the system, although a user can select a
different shell with the
chsh(1)
command. The shell implements a language that has flow control constructs, a
macro facility that provides a variety of features in addition to data
storage, along with built-in history and line editing capabilities. It
incorporates many features to aid interactive use and has the advantage that
the interpretative language is common to both interactive and non-interactive
use (shell scripts). That is, commands can be typed directly to the running
shell or can be put into a file, which can be executed directly by the shell.
If no arguments are present and if the standard input of the shell is connected
to a terminal (or if the
-i
option is set),
the shell is considered an interactive shell. An interactive shell generally
prompts before each command and handles programming and command errors
differently (as described below). When first starting, the shell inspects
argument 0, and if it begins with a dash
(‘
-
’), the shell is also considered a
login shell. This is normally done automatically by the system when the user
first logs in. A login shell first reads commands from the files
/etc/profile and then
.profile in a user's home directory, if
they exist. If the environment variable
ENV
is set on entry to a shell, or is set in the
.profile of a login shell, the shell then
subjects its value to parameter expansion and arithmetic expansion and reads
commands from the named file. Therefore, a user should place commands that are
to be executed only at login time in the
.profile file, and commands that are
executed for every shell inside the
ENV
file. The user can set the
ENV
variable to
some file by placing the following line in the file
.profile in the home directory,
substituting for
.shrc the filename
desired:
ENV=$HOME/.shrc; export
ENV
The first non-option argument specified on the command line will be treated as
the name of a file from which to read commands (a shell script), and the
remaining arguments are set as the positional parameters of the shell
(
$1
,
$2
, etc.). Otherwise, the
shell reads commands from its standard input.
Unlike older versions of
sh
the
ENV
script is only sourced on invocation of
interactive shells. This closes a well-known, and sometimes easily exploitable
security hole related to poorly thought out
ENV
scripts.
All of the single letter options to
sh
have a
corresponding long name, with the exception of
-c
and
-/+o
. These long names are provided next to
the single letter options in the descriptions below. The long name for an
option may be specified as an argument to the
-/+o
option of
sh
. Once the shell is running, the long
name for an option may be specified as an argument to the
-/+o
option of the
set
built-in command (described later in
the section called
Built-in
Commands). Introducing an option with a dash
(‘
-
’) enables the option, while using a
plus (‘
+
’) disables the option. A
“
--
” or plain
‘
-
’ will stop option processing and will
force the remaining words on the command line to be treated as arguments. The
-/+o
and
-c
options do not have long names. They
take arguments and are described after the single letter options.
-a
allexport
- Flag variables for export when assignments are made to them.
-b
notify
- Enable asynchronous notification of background job completion.
(UNIMPLEMENTED)
-C
noclobber
- Do not overwrite existing files with
‘
>
’.
-E
emacs
- Enable the built-in
emacs(1)
command line editor (disables the
-V
option if it has been set; set automatically when interactive on
terminals).
-e
errexit
- Exit immediately if any untested command fails in non-interactive mode.
The exit status of a command is considered to be explicitly tested if the
command is part of the list used to control an
if
,
elif
,
while
, or
until
; if the command is the left hand
operand of an “&&
” or
“||
” operator; or if the command is
a pipeline preceded by the !
keyword.
If a shell function is executed and its exit status is explicitly tested,
all commands of the function are considered to be tested as well.
It is recommended to check for failures explicitly instead of relying on
-e
because it tends to behave in
unexpected ways, particularly in larger scripts.
-f
noglob
- Disable pathname expansion.
-h
trackall
- A do-nothing option for POSIX compliance.
-I
ignoreeof
- Ignore
EOF
's from input when in
interactive mode.
-i
interactive
- Force the shell to behave interactively.
-m
monitor
- Turn on job control (set automatically when interactive). A new process
group is created for each pipeline (called a job). It is possible to
suspend jobs or to have them run in the foreground or in the background.
In a non-interactive shell, this option can be set even if no terminal is
available and is useful to place processes in separate process
groups.
-n
noexec
- If not interactive, read commands but do not execute them. This is useful
for checking the syntax of shell scripts.
-P
physical
- Change the default for the
cd
and
pwd
commands from
-L
(logical directory layout) to
-P
(physical directory layout).
-p
privileged
- Turn on privileged mode. This mode is enabled on startup if either the
effective user or group ID is not equal to the real user or group ID.
Turning this mode off sets the effective user and group IDs to the real
user and group IDs. When this mode is enabled for interactive shells, the
file /etc/suid_profile is sourced
instead of ~/.profile after
/etc/profile is sourced, and the
contents of the
ENV
variable are
ignored.
-s
stdin
- Read commands from standard input (set automatically if no file arguments
are present). This option has no effect when set after the shell has
already started running (i.e., when set with the
set
command).
-T
trapsasync
- When waiting for a child, execute traps immediately. If this option is not
set, traps are executed after the child exits, as specified in
IEEE Std 1003.2
(“POSIX.2”). This nonstandard option is useful for
putting guarding shells around children that block signals. The
surrounding shell may kill the child or it may just return control to the
tty and leave the child alone, like this:
sh -T -c "trap 'exit 1' 2 ; some-blocking-program"
-u
nounset
- Write a message to standard error when attempting to expand a variable, a
positional parameter or the special parameter
! that is not set, and if the shell is
not interactive, exit immediately.
-V
vi
- Enable the built-in
vi(1)
command line editor (disables
-E
if it
has been set).
-v
verbose
- The shell writes its input to standard error as it is read. Useful for
debugging.
-x
xtrace
- Write each command (preceded by the value of the
PS4 variable subjected to parameter
expansion and arithmetic expansion) to standard error before it is
executed. Useful for debugging.
nolog
- Another do-nothing option for POSIX compliance. It only has a long
name.
The
-c
option causes the commands to be read
from the
string operand instead of from the
standard input. Keep in mind that this option only accepts a single string as
its argument, hence multi-word strings must be quoted.
The
-/+o
option takes as its only argument
the long name of an option to be enabled or disabled. For example, the
following two invocations of
sh
both enable
the built-in
emacs(1)
command line editor:
If used without an argument, the
-o
option
displays the current option settings in a human-readable format. If
+o
is used without an argument, the current
option settings are output in a format suitable for re-input into the shell.
The shell reads input in terms of lines from a file and breaks it up into words
at whitespace (blanks and tabs), and at certain sequences of characters called
“operators”, which are special to the shell. There are two types
of operators: control operators and redirection operators (their meaning is
discussed later). The following is a list of valid operators:
- Control operators:
-
- Redirection operators:
-
The character ‘
#
’ introduces a comment if
used at the beginning of a word. The word starting with
‘
#
’ and the rest of the line are
ignored.
ASCII
NUL
characters (character code 0) are
not allowed in shell input.
Quoting is used to remove the special meaning of certain characters or words to
the shell, such as operators, whitespace, keywords, or alias names.
There are four types of quoting: matched single quotes, dollar-single quotes,
matched double quotes, and backslash.
- Single Quotes
- Enclosing characters in single quotes preserves the literal meaning of all
the characters (except single quotes, making it impossible to put
single-quotes in a single-quoted string).
- Dollar-Single Quotes
- Enclosing characters between
$'
and
'
preserves the literal meaning of all characters
except backslashes and single quotes. A backslash introduces a C-style
escape sequence:
- \a
- Alert (ring the terminal bell)
- \b
- Backspace
- \cc
- The control character denoted by
^
c in
stty(1).
If c is a backslash, it must be
doubled.
- \e
- The ESC character (ASCII 0x1b)
- \f
- Formfeed
- \n
- Newline
- \r
- Carriage return
- \t
- Horizontal tab
- \v
- Vertical tab
- \\
- Literal backslash
- \'
- Literal single-quote
- \"
- Literal double-quote
- \nnn
- The byte whose octal value is nnn
(one to three digits)
- \xnn
- The byte whose hexadecimal value is
nn (one or more digits only the last
two of which are used)
- \unnnn
- The Unicode code point nnnn (four
hexadecimal digits)
- \Unnnnnnnn
- The Unicode code point nnnnnnnn
(eight hexadecimal digits)
The sequences for Unicode code points are currently only useful with UTF-8
locales. They reject code point 0 and UTF-16 surrogates.
If an escape sequence would produce a byte with value 0, that byte and the
rest of the string until the matching single-quote are ignored.
Any other string starting with a backslash is an error.
- Double Quotes
- Enclosing characters within double quotes preserves the literal meaning of
all characters except dollar sign
(‘
$
’), backquote
(‘`
’), and backslash
(‘\
’). The backslash inside double
quotes is historically weird. It remains literal unless it precedes the
following characters, which it serves to quote:
- Backslash
- A backslash preserves the literal meaning of the following character, with
the exception of the newline character
(‘
\n
’). A backslash preceding a
newline is treated as a line continuation.
Keywords or reserved words are words that have special meaning to the shell and
are recognized at the beginning of a line and after a control operator. The
following are keywords:
An alias is a name and corresponding value set using the
alias
built-in command. Wherever the
command word of a simple command may occur, and after checking for keywords if
a keyword may occur, the shell checks the word to see if it matches an alias.
If it does, it replaces it in the input stream with its value. For example, if
there is an alias called “
lf
” with the
value “
ls -F
”, then the input
lf foobar
would become
ls -F foobar
Aliases are also recognized after an alias whose value ends with a space or tab.
For example, if there is also an alias called
“
nohup
” with the value
“
nohup
”, then the input
nohup lf foobar
would become
nohup ls -F foobar
Aliases provide a convenient way for naive users to create shorthands for
commands without having to learn how to create functions with arguments. Using
aliases in scripts is discouraged because the command that defines them must
be executed before the code that uses them is parsed. This is fragile and not
portable.
An alias name may be escaped in a command line, so that it is not replaced by
its alias value, by using quoting characters within or adjacent to the alias
name. This is most often done by prefixing an alias name with a backslash to
execute a function, built-in, or normal program with the same name. See the
Quoting subsection.
The shell interprets the words it reads according to a language, the
specification of which is outside the scope of this man page (refer to the BNF
in the
IEEE Std 1003.2
(“POSIX.2”) document). Essentially though, a line is read
and if the first word of the line (or after a control operator) is not a
keyword, then the shell has recognized a simple command. Otherwise, a complex
command or some other special construct may have been recognized.
If a simple command has been recognized, the shell performs the following
actions:
- Leading words of the form
“
name=value
” are stripped off and
assigned to the environment of the simple command (they do not affect
expansions). Redirection operators and their arguments (as described
below) are stripped off and saved for processing.
- The remaining words are expanded as described in the section called
Word Expansions, and
the first remaining word is considered the command name and the command is
located. The remaining words are considered the arguments of the command.
If no command name resulted, then the
“
name=value
” variable assignments
recognized in 1) affect the current shell.
- Redirections are performed as described in the next section.
Redirections are used to change where a command reads its input or sends its
output. In general, redirections open, close, or duplicate an existing
reference to a file. The overall format used for redirection is:
The
redir-op is one of the redirection
operators mentioned previously. The following gives some examples of how these
operators can be used. Note that stdin and stdout are commonly used
abbreviations for standard input and standard output respectively.
- [
n
]>
file
- redirect stdout (or file descriptor n) to
file
- [
n
]>|
file
- same as above, but override the
-C
option
- [
n
]>>
file
- append stdout (or file descriptor n) to
file
- [
n
]<
file
- redirect stdin (or file descriptor n)
from file
- [
n
]<>
file
- redirect stdin (or file descriptor n) to
and from file
- [
n1
]<&
n2
- duplicate stdin (or file descriptor n1)
from file descriptor n2
- [
n
]<&-
- close stdin (or file descriptor n)
- [
n1
]>&
n2
- duplicate stdout (or file descriptor n1)
to file descriptor n2
- [
n
]>&-
- close stdout (or file descriptor n)
The following redirection is often called a “here-document”.
[n
]<<
delimiter
here-doc-text
...
delimiter
All the text on successive lines up to the delimiter is saved away and made
available to the command on standard input, or file descriptor
n if it is specified. If the
delimiter as specified on the initial line is
quoted, then the
here-doc-text is treated
literally, otherwise the text is subjected to parameter expansion, command
substitution, and arithmetic expansion (as described in the section on
Word Expansions). If the
operator is “
<<-
” instead of
“
<<
”, then leading tabs in the
here-doc-text are stripped.
There are three types of commands: shell functions, built-in commands, and
normal programs. The command is searched for (by name) in that order. The
three types of commands are all executed in a different way.
When a shell function is executed, all of the shell positional parameters
(except
$0
, which remains unchanged) are set to the
arguments of the shell function. The variables which are explicitly placed in
the environment of the command (by placing assignments to them before the
function name) are made local to the function and are set to the values given.
Then the command given in the function definition is executed. The positional
parameters are restored to their original values when the command completes.
This all occurs within the current shell.
Shell built-in commands are executed internally to the shell, without spawning a
new process. There are two kinds of built-in commands: regular and special.
Assignments before special builtins persist after they finish executing and
assignment errors, redirection errors and certain operand errors cause a
script to be aborted. Special builtins cannot be overridden with a function.
Both regular and special builtins can affect the shell in ways normal programs
cannot.
Otherwise, if the command name does not match a function or built-in command,
the command is searched for as a normal program in the file system (as
described in the next section). When a normal program is executed, the shell
runs the program, passing the arguments and the environment to the program. If
the program is not a normal executable file (i.e., if it does not begin with
the “magic number” whose ASCII representation is
“
#!
”, resulting in an
ENOEXEC
return value from
execve(2))
but appears to be a text file, the shell will run a new instance of
sh
to interpret it.
Note that previous versions of this document and the source code itself
misleadingly and sporadically refer to a shell script without a magic number
as a “shell procedure”.
When locating a command, the shell first looks to see if it has a shell function
by that name. Then it looks for a built-in command by that name. If a built-in
command is not found, one of two things happen:
- Command names containing a slash are simply executed without performing
any searches.
- The shell searches each entry in the PATH
variable in turn for the command. The value of the
PATH variable should be a series of
entries separated by colons. Each entry consists of a directory name. The
current directory may be indicated implicitly by an empty directory name,
or explicitly by a single period.
Each command has an exit status that can influence the behavior of other shell
commands. The paradigm is that a command exits with zero for normal or
success, and non-zero for failure, error, or a false indication. The man page
for each command should indicate the various exit codes and what they mean.
Additionally, the built-in commands return exit codes, as does an executed
shell function.
If a command is terminated by a signal, its exit status is greater than 128. The
signal name can be found by passing the exit status to
kill
-l
.
If there is no command word, the exit status is the exit status of the last
command substitution executed, or zero if the command does not contain any
command substitutions.
Complex commands are combinations of simple commands with control operators or
keywords, together creating a larger complex command. More generally, a
command is one of the following:
- simple command
- pipeline
- list or compound-list
- compound command
- function definition
Unless otherwise stated, the exit status of a command is that of the last simple
command executed by the command, or zero if no simple command was executed.
A pipeline is a sequence of one or more commands separated by the control
operator ‘
|
’. The standard output of all
but the last command is connected to the standard input of the next command.
The standard output of the last command is inherited from the shell, as usual.
The format for a pipeline is:
[
!
]
command1
[
|
command2
...
]
The standard output of
command1 is connected to
the standard input of
command2. The standard
input, standard output, or both of a command is considered to be assigned by
the pipeline before any redirection specified by redirection operators that
are part of the command.
Note that unlike some other shells,
sh
executes each process in a pipeline with more than one command in a subshell
environment and as a child of the
sh
process.
If the pipeline is not in the background (discussed later), the shell waits for
all commands to complete.
If the keyword
!
does not precede the
pipeline, the exit status is the exit status of the last command specified in
the pipeline. Otherwise, the exit status is the logical NOT of the exit status
of the last command. That is, if the last command returns zero, the exit
status is 1; if the last command returns greater than zero, the exit status is
zero.
Because pipeline assignment of standard input or standard output or both takes
place before redirection, it can be modified by redirection. For example:
command1 2>&1 |
command2
sends both the standard output and standard error of
command1 to the standard input of
command2.
A ‘
;
’ or newline terminator causes the
preceding AND-OR-list (described below in the section called
Short-Circuit
List Operators) to be executed sequentially; an
‘
&
’ causes asynchronous execution of
the preceding AND-OR-list.
If a command is terminated by the control operator ampersand
(‘
&
’), the shell executes the
command in a subshell environment (see
Grouping Commands
Together below) and asynchronously; the shell does not wait for the
command to finish before executing the next command.
The format for running a command in background is:
command1
&
[
command2
&
...
]
If the shell is not interactive, the standard input of an asynchronous command
is set to
/dev/null.
The exit status is zero.
A list is a sequence of zero or more commands separated by newlines, semicolons,
or ampersands, and optionally terminated by one of these three characters. The
commands in a list are executed in the order they are written. If command is
followed by an ampersand, the shell starts the command and immediately
proceeds onto the next command; otherwise it waits for the command to
terminate before proceeding to the next one.
“
&&
” and
“
||
” are AND-OR list operators.
“
&&
” executes the first command,
and then executes the second command if the exit status of the first command
is zero. “
||
” is similar, but executes
the second command if the exit status of the first command is nonzero.
“
&&
” and
“
||
” both have the same priority.
The syntax of the
if
command is:
if
list
then
list
[elif
list then
list
] ...
[else
list
]
fi
The exit status is that of selected
then
or
else
list, or zero if no list was selected.
The syntax of the
while
command is:
The two lists are executed repeatedly while the exit status of the first list is
zero. The
until
command is similar, but has
the word
until
in place of
while
, which causes it to repeat until the
exit status of the first list is zero.
The exit status is that of the last execution of the second list, or zero if it
was never executed.
The syntax of the
for
command is:
for
variable [in
word ...
]
do
list
done
If
in
and the following words are omitted,
in
"$@"
is used instead. The words are expanded, and then the list is executed
repeatedly with the variable set to each word in turn. The
do
and
done
commands may be replaced with
‘
{
’ and
‘
}
’.
The syntax of the
break
and
continue
commands is:
The
break
command terminates the
num innermost
for
or
while
loops. The
continue
command continues with the next
iteration of the innermost loop. These are implemented as special built-in
commands.
The syntax of the
case
command is:
case
word in
pattern) list ;;
...
esac
The pattern can actually be one or more patterns (see
Shell Patterns described
later), separated by ‘
|
’ characters.
Tilde expansion, parameter expansion, command substitution, arithmetic
expansion and quote removal are applied to the word. Then, each pattern is
expanded in turn using tilde expansion, parameter expansion, command
substitution and arithmetic expansion and the expanded form of the word is
checked against it. If a match is found, the corresponding list is executed.
If the selected list is terminated by the control operator
‘
;&
’ instead of
‘
;;
’, execution continues with the next
list, continuing until a list terminated with
‘
;;
’ or the end of the
case
command.
Commands may be grouped by writing either
or
The first form executes the commands in a subshell environment. A subshell
environment has its own copy of:
- The current working directory as set by
cd
.
- The file creation mask as set by
umask
.
- Resource limits as set by
ulimit
.
- References to open files.
- Traps as set by
trap
.
- Known jobs.
- Positional parameters and variables.
- Shell options.
- Shell functions.
- Shell aliases.
These are copied from the parent shell environment, except that trapped (but not
ignored) signals are reset to the default action and known jobs are cleared.
Any changes do not affect the parent shell environment.
A subshell environment may be implemented as a child process or differently. If
job control is enabled in an interactive shell, commands grouped in
parentheses can be suspended and continued as a unit.
For compatibility with other shells, two open parentheses in sequence should be
separated by whitespace.
The second form never forks another shell, so it is slightly more efficient.
Grouping commands together this way allows the user to redirect their output
as though they were one program:
{ echo -n "hello"; echo " world"; } > greeting
The syntax of a function definition is
name
( )
command
A function definition is an executable statement; when executed it installs a
function named
name and returns an exit
status of zero. The
command is normally a
list enclosed between ‘
{
’ and
‘
}
’.
Variables may be declared to be local to a function by using the
local
command. This should appear as the
first statement of a function, and the syntax is:
The
local
command is implemented as a
built-in command. The exit status is zero unless the command is not in a
function or a variable name is invalid.
When a variable is made local, it inherits the initial value and exported and
readonly flags from the variable with the same name in the surrounding scope,
if there is one. Otherwise, the variable is initially unset. The shell uses
dynamic scoping, so that if the variable
x is
made local to function
f, which then calls
function
g, references to the variable
x made inside
g
will refer to the variable
x declared inside
f, not to the global variable named
x.
The only special parameter that can be made local is
‘
-
’. Making
‘
-
’ local causes any shell options
(including those that only have long names) that are changed via the
set
command inside the function to be
restored to their original values when the function returns.
The syntax of the
return
command is
It terminates the current executional scope, returning from the closest nested
function or sourced script; if no function or sourced script is being
executed, it exits the shell instance. The
return
command is implemented as a special
built-in command.
The shell maintains a set of parameters. A parameter denoted by a name
(consisting solely of alphabetics, numerics, and underscores, and starting
with an alphabetic or an underscore) is called a variable. When starting up,
the shell turns all environment variables with valid names into shell
variables. New variables can be set using the form
name=value
A parameter can also be denoted by a number or a special character as explained
below.
Assignments are expanded differently from other words: tilde expansion is also
performed after the equals sign and after any colon and usernames are also
terminated by colons, and field splitting and pathname expansion are not
performed.
This special expansion applies not only to assignments that form a simple
command by themselves or precede a command word, but also to words passed to
the
export
,
local
or
readonly
built-in commands that have this
form. For this, the builtin's name must be literal (not the result of an
expansion) and may optionally be preceded by one or more literal instances of
command
without options.
A positional parameter is a parameter denoted by a number greater than zero. The
shell sets these initially to the values of its command line arguments that
follow the name of the shell script. The
set
built-in command can also be used to
set or reset them.
Special parameters are parameters denoted by a single special character or the
digit zero. They are shown in the following list, exactly as they would appear
in input typed by the user or in the source of a shell script.
$*
- Expands to the positional parameters, starting from one. When the
expansion occurs within a double-quoted string it expands to a single
field with the value of each parameter separated by the first character of
the IFS variable, or by a space if
IFS is unset.
$@
- Expands to the positional parameters, starting from one. When the
expansion occurs within double-quotes, each positional parameter expands
as a separate argument. If there are no positional parameters, the
expansion of
@
generates zero arguments, even when
@
is double-quoted. What this basically means, for
example, is if $1
is
“abc
” and $2
is “def ghi
”, then
"$@"
expands to the two arguments:
$#
- Expands to the number of positional parameters.
$?
- Expands to the exit status of the most recent pipeline.
$-
- (hyphen) Expands to the current option flags (the single-letter option
names concatenated into a string) as specified on invocation, by the
set
built-in command, or implicitly by
the shell.
$$
- Expands to the process ID of the invoked shell. A subshell retains the
same value of $ as its parent.
$!
- Expands to the process ID of the most recent background command executed
from the current shell. For a pipeline, the process ID is that of the last
command in the pipeline. If this parameter is referenced, the shell will
remember the process ID and its exit status until the
wait
built-in command reports
completion of the process.
$0
- (zero) Expands to the name of the shell script if passed on the command
line, the name operand if given (with
-c
) or otherwise argument 0 passed to
the shell.
The following variables are set by the shell or have special meaning to it:
- CDPATH
- The search path used with the
cd
built-in.
- EDITOR
- The fallback editor used with the
fc
built-in. If not set, the default editor is
ed(1).
- FCEDIT
- The default editor used with the
fc
built-in.
- HISTSIZE
- The number of previous commands that are accessible.
- HOME
- The user's home directory, used in tilde expansion and as a default
directory for the
cd
built-in.
- IFS
- Input Field Separators. This is initialized at startup to
⟨space⟩, ⟨tab⟩, and ⟨newline⟩ in
that order. This value also applies if
IFS is unset, but not if it is set to the
empty string. See the
White Space
Splitting section for more details.
- LINENO
- The current line number in the script or function.
- MAIL
- The name of a mail file, that will be checked for the arrival of new mail.
Overridden by MAILPATH.
- MAILPATH
- A colon (‘
:
’) separated list of file
names, for the shell to check for incoming mail. This variable overrides
the MAIL setting. There is a maximum of
10 mailboxes that can be monitored at once.
- OPTIND
- The index of the next argument to be processed by
getopts
. This is initialized to 1 at
startup.
- PATH
- The default search path for executables. See the
Path Search section for
details.
- PPID
- The parent process ID of the invoked shell. This is set at startup unless
this variable is in the environment. A later change of parent process ID
is not reflected. A subshell retains the same value of
PPID.
- PS1
- The primary prompt string, which defaults to “
$
”, unless you are the superuser, in which case it defaults
to “#
”.
PS1 may include any of the following
formatting sequences, which are replaced by the given information:
\H
- This system's fully-qualified hostname (FQDN).
\h
- This system's hostname.
\W
- The final component of the current working directory.
\w
- The entire path of the current working directory.
\$
- Superuser status. “
$
” for
normal users and “#
” for
superusers.
\\
- A literal backslash.
- PS2
- The secondary prompt string, which defaults to
“
>
”.
PS2 may include any of the formatting
sequences from PS1.
- PS4
- The prefix for the trace output (if
-x
is active). The default is “+
”.
This clause describes the various expansions that are performed on words. Not
all expansions are performed on every word, as explained later.
Tilde expansions, parameter expansions, command substitutions, arithmetic
expansions, and quote removals that occur within a single word expand to a
single field. It is only field splitting or pathname expansion that can create
multiple fields from a single word. The single exception to this rule is the
expansion of the special parameter
@ within
double-quotes, as was described above.
The order of word expansion is:
- Tilde Expansion, Parameter Expansion, Command Substitution, Arithmetic
Expansion (these all occur at the same time).
- Field Splitting is performed on fields generated by step (1) unless the
IFS variable is null.
- Pathname Expansion (unless the
-f
option is in effect).
- Quote Removal.
The ‘
$
’ character is used to introduce
parameter expansion, command substitution, or arithmetic expansion.
A word beginning with an unquoted tilde character
(‘
~
’) is subjected to tilde expansion.
All the characters up to a slash (‘
/
’)
or the end of the word are treated as a username and are replaced with the
user's home directory. If the username is missing (as in
~/foobar), the tilde is replaced with the
value of the
HOME variable (the current
user's home directory).
The format for parameter expansion is as follows:
${
expression}
where
expression consists of all characters
until the matching ‘
}
’. Any
‘
}
’ escaped by a backslash or within a
single-quoted or double-quoted string, and characters in embedded arithmetic
expansions, command substitutions, and variable expansions, are not examined
in determining the matching ‘
}
’. If the
variants with ‘
+
’,
‘
-
’,
‘
=
’ or
‘
?
’ occur within a double-quoted string,
as an extension there may be unquoted parts (via double-quotes inside the
expansion); ‘
}
’ within such parts are
also not examined in determining the matching
‘
}
’.
The simplest form for parameter expansion is:
${
parameter}
The value, if any, of
parameter is substituted.
The parameter name or symbol can be enclosed in braces, which are optional
except for positional parameters with more than one digit or when parameter is
followed by a character that could be interpreted as part of the name. If a
parameter expansion occurs inside double-quotes:
- Field splitting is not performed on the results of the expansion, with the
exception of the special parameter
@.
- Pathname expansion is not performed on the results of the expansion.
In addition, a parameter expansion can be modified by using one of the following
formats.
${
parameter:-
word}
- Use Default Values. If parameter is unset
or null, the expansion of word is
substituted; otherwise, the value of
parameter is substituted.
${
parameter:=
word}
- Assign Default Values. If parameter is
unset or null, the expansion of word is
assigned to parameter. In all cases, the
final value of parameter is substituted.
Quoting inside word does not prevent
field splitting or pathname expansion. Only variables, not positional
parameters or special parameters, can be assigned in this way.
${
parameter:?
[word
]}
- Indicate Error if Null or Unset. If
parameter is unset or null, the expansion
of word (or a message indicating it is
unset if word is omitted) is written to
standard error and the shell exits with a nonzero exit status. Otherwise,
the value of parameter is substituted. An
interactive shell need not exit.
${
parameter:+
word}
- Use Alternate Value. If parameter is
unset or null, null is substituted; otherwise, the expansion of
word is substituted.
In the parameter expansions shown previously, use of the colon in the format
results in a test for a parameter that is unset or null; omission of the colon
results in a test for a parameter that is only unset.
The
word inherits the type of quoting
(unquoted, double-quoted or here-document) from the surroundings, with the
exception that a backslash that quotes a closing brace is removed during quote
removal.
${#
parameter}
- String Length. The length in characters of the value of
parameter.
The following four varieties of parameter expansion provide for substring
processing. In each case, pattern matching notation (see
Shell Patterns), rather
than regular expression notation, is used to evaluate the patterns. If
parameter is one of the special parameters
*
or
@, the result of the expansion is
unspecified. Enclosing the full parameter expansion string in double-quotes
does not cause the following four varieties of pattern characters to be
quoted, whereas quoting characters within the braces has this effect.
${
parameter%
word}
- Remove Smallest Suffix Pattern. The word
is expanded to produce a pattern. The parameter expansion then results in
parameter, with the smallest portion of
the suffix matched by the pattern deleted.
${
parameter%%
word}
- Remove Largest Suffix Pattern. The word
is expanded to produce a pattern. The parameter expansion then results in
parameter, with the largest portion of
the suffix matched by the pattern deleted.
${
parameter#
word}
- Remove Smallest Prefix Pattern. The word
is expanded to produce a pattern. The parameter expansion then results in
parameter, with the smallest portion of
the prefix matched by the pattern deleted.
${
parameter##
word}
- Remove Largest Prefix Pattern. The word
is expanded to produce a pattern. The parameter expansion then results in
parameter, with the largest portion of
the prefix matched by the pattern deleted.
Command substitution allows the output of a command to be substituted in place
of the command name itself. Command substitution occurs when the command is
enclosed as follows:
$(
command)
or the backquoted version:
`
command`
The shell expands the command substitution by executing command and replacing
the command substitution with the standard output of the command, removing
sequences of one or more newlines at the end of the substitution. Embedded
newlines before the end of the output are not removed; however, during field
splitting, they may be translated into spaces depending on the value of
IFS and the quoting that is in effect. The
command is executed in a subshell environment, except that the built-in
commands
jobid
,
jobs
, and
trap
return information about the parent
shell environment and
times
returns
information about the same process if they are the only command in a command
substitution.
If a command substitution of the
$(
form begins with a
subshell, the
$(
and
(
must be
separated by whitespace to avoid ambiguity with arithmetic expansion.
Arithmetic expansion provides a mechanism for evaluating an arithmetic
expression and substituting its value. The format for arithmetic expansion is
as follows:
$((
expression))
The
expression is treated as if it were in
double-quotes, except that a double-quote inside the expression is not treated
specially. The shell expands all tokens in the
expression for parameter expansion, command
substitution, arithmetic expansion and quote removal.
The allowed expressions are a subset of C expressions, summarized below.
The result of the expression is substituted in decimal.
In certain contexts, after parameter expansion, command substitution, and
arithmetic expansion the shell scans the results of expansions and
substitutions that did not occur in double-quotes for field splitting and
multiple fields can result.
Characters in
IFS that are whitespace
(⟨space⟩, ⟨tab⟩, and ⟨newline⟩) are
treated differently from other characters in
IFS.
Whitespace in
IFS at the beginning or end of a
word is discarded.
Subsequently, a field is delimited by either
- a non-whitespace character in IFS with
any whitespace in IFS surrounding it,
or
- one or more whitespace characters in
IFS.
If a word ends with a non-whitespace character in
IFS, there is no empty field after this
character.
If no field is delimited, the word is discarded. In particular, if a word
consists solely of an unquoted substitution and the result of the substitution
is null, it is removed by field splitting even if
IFS is null.
Unless the
-f
option is set, file name
generation is performed after word splitting is complete. Each word is viewed
as a series of patterns, separated by slashes. The process of expansion
replaces the word with the names of all existing files whose names can be
formed by replacing each pattern with a string that matches the specified
pattern. There are two restrictions on this: first, a pattern cannot match a
string containing a slash, and second, a pattern cannot match a string
starting with a period unless the first character of the pattern is a period.
The next section describes the patterns used for Pathname Expansion, the four
varieties of parameter expansion for substring processing and the
case
command.
A pattern consists of normal characters, which match themselves, and
meta-characters. The meta-characters are
‘
*
’,
‘
?
’, and
‘
[
’. These characters lose their special
meanings if they are quoted. When command or variable substitution is
performed and the dollar sign or back quotes are not double-quoted, the value
of the variable or the output of the command is scanned for these characters
and they are turned into meta-characters.
An asterisk (‘
*
’) matches any string of
characters. A question mark (‘
?
’)
matches any single character. A left bracket
(‘
[
’) introduces a character class. The
end of the character class is indicated by a
‘
]
’; if the
‘
]
’ is missing then the
‘
[
’ matches a
‘
[
’ rather than introducing a character
class. A character class matches any of the characters between the square
brackets. A locale-dependent range of characters may be specified using a
minus sign. A named class of characters (see
wctype(3))
may be specified by surrounding the name with
‘
[:
’ and
‘
:]
’. For example,
‘
[[:alpha:]]
’ is a shell pattern that
matches a single letter. The character class may be complemented by making an
exclamation point (‘
!
’) the first
character of the character class. A caret
(‘
^
’) has the same effect but is
non-standard.
To include a ‘
]
’ in a character class,
make it the first character listed (after the
‘
!
’ or
‘
^
’, if any). To include a
‘
-
’, make it the first or last character
listed.
This section lists the built-in commands.
:
- A null command that returns a 0 (true) exit value.
.
file
- The commands in the specified file are read and executed by the shell. The
return
command may be used to return to
the .
command's caller. If
file contains any
‘/
’ characters, it is used as is.
Otherwise, the shell searches the PATH
for the file. If it is not found in the
PATH, it is sought in the current working
directory.
[
- A built-in equivalent of
test(1).
alias
[]
- If
name=string
is specified, the shell defines the alias
name with value
string. If just
name is specified, the value of the alias
name is printed. With no arguments, the
alias
built-in command prints the names
and values of all defined aliases (see
unalias
). Alias values are written with
appropriate quoting so that they are suitable for re-input to the shell.
Also see the Aliases
subsection.
bg
[job ...
]
- Continue the specified jobs (or the current job if no jobs are given) in
the background.
bind
[-aeklrsv
]
[]
- List or alter key bindings for the line editor. This command is documented
in
editrc(5).
break
[num
]
- See the
Flow-Control
Constructs subsection.
builtin
cmd
[arg ...
]
- Execute the specified built-in command,
cmd. This is useful when the user wishes
to override a shell function with the same name as a built-in
command.
cd
[-L
|
-P
]
[-e
]
[directory
]
-
cd
-
- Switch to the specified directory, to the
directory specified in the HOME
environment variable if no directory is
specified or to the directory specified in the
OLDPWD environment variable if
directory is
-
. If
directory does not begin with
/,
., or
.., then the directories listed in the
CDPATH variable will be searched for the
specified directory. If
CDPATH is unset, the current directory is
searched. The format of CDPATH is the
same as that of PATH. In an interactive
shell, the cd
command will print out
the name of the directory that it actually switched to if the
CDPATH mechanism was used or if
directory was
-
.
If the -P
option is specified,
.. is handled physically and symbolic
links are resolved before .. components
are processed. If the -L
option is
specified, .. is handled logically.
This is the default.
The -e
option causes
cd
to return exit status 1 if the full
pathname of the new directory cannot be determined reliably or at all.
Normally this is not considered an error, although a warning is printed.
If changing the directory fails, the exit status is greater than 1. If the
directory is changed, the exit status is 0, or also 1 if
-e
was given.
chdir
- A synonym for the
cd
built-in
command.
command
[-p
]
[]
-
command
[-p
]
-v
utility
-
command
[-p
]
-V
utility
- The first form of invocation executes the specified
utility, ignoring shell functions in the
search. If utility is a special builtin,
it is executed as if it were a regular builtin.
If the
-p
option is specified, the
command search is performed using a default value of
PATH that is guaranteed to find all of
the standard utilities.
If the -v
option is specified,
utility is not executed but a description
of its interpretation by the shell is printed. For ordinary commands the
output is the path name; for shell built-in commands, shell functions and
keywords only the name is written. Aliases are printed as
“alias
name=value”.
The -V
option is identical to
-v
except for the output. It prints
“utility
is
description” where
description is either the path name to
utility, a special shell builtin, a shell
builtin, a shell function, a shell keyword or an alias for
value.
continue
[num
]
- See the
Flow-Control
Constructs subsection.
echo
[-e
|
-n
]
[string ...
]
- Print a space-separated list of the arguments to the standard output and
append a newline character.
-n
- Suppress the output of the trailing newline.
-e
- Process C-style backslash escape sequences. The
echo
command understands the
following character escapes:
- \a
- Alert (ring the terminal bell)
- \b
- Backspace
- \c
- Suppress the trailing newline (this has the side-effect of
truncating the line if it is not the last character)
- \e
- The ESC character (ASCII 0x1b)
- \f
- Formfeed
- \n
- Newline
- \r
- Carriage return
- \t
- Horizontal tab
- \v
- Vertical tab
- \\
- Literal backslash
- \0nnn
- (Zero) The character whose octal value is
nnn
If string is not enclosed in quotes
then the backslash itself must be escaped with a backslash to protect
it from the shell. For example
$ echo -e "a\vb"
a
b
$ echo -e a\\vb
a
b
$ echo -e "a\\b"
a\b
$ echo -e a\\\\b
a\b
Only one of the -e
and
-n
options may be specified.
eval
string ...
- Concatenate all the arguments with spaces. Then re-parse and execute the
command.
exec
[]
- Unless command is omitted, the shell
process is replaced with the specified program (which must be a real
program, not a shell built-in command or function). Any redirections on
the
exec
command are marked as
permanent, so that they are not undone when the
exec
command finishes.
exit
[exitstatus
]
- Terminate the shell process. If
exitstatus is given it is used as the
exit status of the shell. Otherwise, if the shell is executing an
EXIT
trap, the exit status of the last
command before the trap is used; if the shell is executing a trap for a
signal, the shell exits by resending the signal to itself. Otherwise, the
exit status of the preceding command is used. The exit status should be an
integer between 0 and 255.
export
name ...
-
export
[-p
]
- The specified names are exported so that they will appear in the
environment of subsequent commands. The only way to un-export a variable
is to
unset
it. The shell allows the
value of a variable to be set at the same time as it is exported by
writing
export
name=value
With no arguments the export
command
lists the names of all exported variables. If the
-p
option is specified, the exported
variables are printed as “export
name=value”
lines, suitable for re-input to the shell.
false
- A null command that returns a non-zero (false) exit value.
fc
[-e
editor
]
[]
-
fc
-l
[-nr
]
[]
-
fc
-s
[old=new
]
[first
]
- The
fc
built-in command lists, or edits
and re-executes, commands previously entered to an interactive shell.
-e
editor
- Use the editor named by editor to
edit the commands. The editor string
is a command name, subject to search via the
PATH variable. The value in the
FCEDIT variable is used as a default
when
-e
is not specified. If
FCEDIT is null or unset, the value of
the EDITOR variable is used. If
EDITOR is null or unset,
ed(1)
is used as the editor.
-l
(ell)
- List the commands rather than invoking an editor on them. The commands
are written in the sequence indicated by the
first and
last operands, as affected by
-r
, with each command preceded by
the command number.
-n
- Suppress command numbers when listing with
-l
.
-r
- Reverse the order of the commands listed (with
-l
) or edited (with neither
-l
nor
-s
).
-s
- Re-execute the command without invoking an editor.
- first
-
- last
- Select the commands to list or edit. The number of previous commands
that can be accessed are determined by the value of the
HISTSIZE variable. The value of
first or
last or both are one of the
following:
- [
+
]num
- A positive number representing a command number; command numbers
can be displayed with the
-l
option.
-
num
- A negative decimal number representing the command that was
executed num of commands
previously. For example, -1 is the immediately previous
command.
- string
- A string indicating the most recently entered command that begins
with that string. If the
old=new
operand is not also specified with
-s
, the string form of the
first operand cannot contain an embedded equal sign.
The following variables affect the execution of
fc
:
- FCEDIT
- Name of the editor to use for history editing.
- HISTSIZE
- The number of previous commands that are accessible.
fg
[job
]
- Move the specified job or the current job
to the foreground.
getopts
optstring var
- The POSIX
getopts
command. The
getopts
command deprecates the older
getopt(1)
command. The first argument should be a series of letters, each possibly
followed by a colon which indicates that the option takes an argument. The
specified variable is set to the parsed option. The index of the next
argument is placed into the shell variable
OPTIND. If an option takes an argument,
it is placed into the shell variable
OPTARG. If an invalid option is
encountered, var is set to
‘?
’. It returns a false value (1)
when it encounters the end of the options. A new set of arguments may be
parsed by assigning OPTIND=1
.
hash
[-rv
]
[command ...
]
- The shell maintains a hash table which remembers the locations of
commands. With no arguments whatsoever, the
hash
command prints out the contents of
this table.
With arguments, the hash
command removes
each specified command from the hash
table (unless they are functions) and then locates it. With the
-v
option,
hash
prints the locations of the
commands as it finds them. The -r
option causes the hash
command to
delete all the entries in the hash table except for functions.
jobid
[job
]
- Print the process IDs of the processes in the specified
job. If the
job argument is omitted, use the current
job.
jobs
[-lps
]
[job ...
]
- Print information about the specified jobs, or all jobs if no
job argument is given. The information
printed includes job ID, status and command name.
If the
-l
option is specified, the PID of
each job is also printed. If the -p
option is specified, only the process IDs for the process group leaders
are printed, one per line. If the -s
option is specified, only the PIDs of the job commands are printed, one
per line.
kill
- A built-in equivalent of
kill(1)
that additionally supports sending signals to jobs.
local
[variable ...
]
[-
]
- See the Functions
subsection.
printf
- A built-in equivalent of
printf(1).
pwd
[-L
|
-P
]
- Print the path of the current directory. The built-in command may differ
from the program of the same name because the built-in command remembers
what the current directory is rather than recomputing it each time. This
makes it faster. However, if the current directory is renamed, the
built-in version of
pwd(1)
will continue to print the old name for the directory.
If the
-P
option is specified, symbolic
links are resolved. If the -L
option is
specified, the shell's notion of the current directory is printed
(symbolic links are not resolved). This is the default.
read
[-p
prompt
]
[-t
timeout
]
[-er
]
variable ...
- The prompt is printed if the
-p
option is specified and the standard
input is a terminal. Then a line is read from the standard input. The
trailing newline is deleted from the line and the line is split as
described in the section on
White
Space Splitting (Field Splitting) above, and the pieces are assigned
to the variables in order. If there are more pieces than variables, the
remaining pieces (along with the characters in
IFS that separated them) are assigned to
the last variable. If there are more variables than pieces, the remaining
variables are assigned the null string.
Backslashes are treated specially, unless the
-r
option is specified. If a backslash
is followed by a newline, the backslash and the newline will be deleted.
If a backslash is followed by any other character, the backslash will be
deleted and the following character will be treated as though it were not
in IFS, even if it is.
If the -t
option is specified and the
timeout elapses before a complete line of
input is supplied, the read
command
will return an exit status as if terminated by
SIGALRM
without assigning any values.
The timeout value may optionally be
followed by one of ‘s
’,
‘m
’ or
‘h
’ to explicitly specify seconds,
minutes or hours. If none is supplied,
‘s
’ is assumed.
The -e
option exists only for backward
compatibility with older scripts.
The exit status is 0 on success, 1 on end of file, between 2 and 128 if an
error occurs and greater than 128 if a trapped signal interrupts
read
.
readonly
[-p
]
[name ...
]
- Each specified name is marked as read
only, so that it cannot be subsequently modified or unset. The shell
allows the value of a variable to be set at the same time as it is marked
read only by using the following form:
readonly
name=value
With no arguments the readonly
command
lists the names of all read only variables. If the
-p
option is specified, the read-only
variables are printed as
“readonly
name=value”
lines, suitable for re-input to the shell.
return
[exitstatus
]
- See the Functions
subsection.
set
[-/+abCEefIimnpTuVvx
]
[-/+o
longname
]
[-c
string
]
[--
arg ...
]
- The
set
command performs three
different functions:
- With no arguments, it lists the values of all shell variables.
- If options are given, either in short form or using the long
“
-/+o
longname” form, it sets or
clears the specified options as described in the section called
Argument
List Processing.
- If the “
--
” option is
specified, set
will replace the
shell's positional parameters with the subsequent arguments. If no
arguments follow the
“--
” option, all the
positional parameters will be cleared, which is equivalent to
executing the command “shift
$#
”. The
“--
” flag may be
omitted when specifying arguments to be used as positional replacement
parameters. This is not recommended, because the first argument may
begin with a dash (‘-
’) or a
plus (‘+
’), which the
set
command will interpret as a
request to enable or disable options.
setvar
variable value
- Assigns the specified value to the
specified variable. The
setvar
command is intended to be used
in functions that assign values to variables whose names are passed as
parameters. In general it is better to write
“variable=value”
rather than using setvar
.
shift
[n
]
- Shift the positional parameters n times,
or once if n is not specified. A shift
sets the value of
$1
to the value of
$2
, the value of $2
to the
value of $3
, and so on, decreasing the value of
$#
by one. For portability, shifting if there are
zero positional parameters should be avoided, since the shell may
abort.
test
- A built-in equivalent of
test(1).
times
- Print the amount of time spent executing the shell process and its
children. The first output line shows the user and system times for the
shell process itself, the second one contains the user and system times
for the children.
trap
[action
]
signal ...
-
trap
-l
- Cause the shell to parse and execute
action when any specified
signal is received. The signals are
specified by name or number. In addition, the pseudo-signal
EXIT
may be used to specify an
action that is performed when the shell
terminates. The action may be an empty
string or a dash (‘-
’); the former
causes the specified signal to be ignored and the latter causes the
default action to be taken. Omitting the
action and using only signal numbers is
another way to request the default action. In a subshell or utility
environment, the shell resets trapped (but not ignored) signals to the
default action. The trap
command has no
effect on signals that were ignored on entry to the shell.
Option -l
causes the
trap
command to display a list of valid
signal names.
true
- A null command that returns a 0 (true) exit value.
type
[name ...
]
- Interpret each name as a command and
print the resolution of the command search. Possible resolutions are:
shell keyword, alias, special shell builtin, shell builtin, command,
tracked alias and not found. For aliases the alias expansion is printed;
for commands and tracked aliases the complete pathname of the command is
printed.
ulimit
[-HSabcdfklmnopstuvw
]
[limit
]
- Set or display resource limits (see
getrlimit(2)).
If limit is specified, the named resource
will be set; otherwise the current resource value will be displayed.
If
-H
is specified, the hard limits will
be set or displayed. While everybody is allowed to reduce a hard limit,
only the superuser can increase it. The
-S
option specifies the soft limits
instead. When displaying limits, only one of
-S
or
-H
can be given. The default is to
display the soft limits, and to set both the hard and the soft limits.
Option -a
causes the
ulimit
command to display all
resources. The parameter limit is not
acceptable in this mode.
The remaining options specify which resource value is to be displayed or
modified. They are mutually exclusive.
-b
sbsize
- The maximum size of socket buffer usage, in bytes.
-c
coredumpsize
- The maximal size of core dump files, in 512-byte blocks. Setting
coredumpsize to 0 prevents core dump
files from being created.
-d
datasize
- The maximal size of the data segment of a process, in kilobytes.
-f
filesize
- The maximal size of a file, in 512-byte blocks.
-k
kqueues
- The maximal number of kqueues (see
kqueue(2))
for this user ID.
-l
lockedmem
- The maximal size of memory that can be locked by a process, in
kilobytes.
-m
memoryuse
- The maximal resident set size of a process, in kilobytes.
-n
nofiles
- The maximal number of descriptors that could be opened by a
process.
-o
umtxp
- The maximal number of process-shared locks (see
pthread(3))
for this user ID.
-p
pseudoterminals
- The maximal number of pseudo-terminals for this user ID.
-s
stacksize
- The maximal size of the stack segment, in kilobytes.
-t
time
- The maximal amount of CPU time to be used by each process, in
seconds.
-u
userproc
- The maximal number of simultaneous processes for this user ID.
-v
virtualmem
- The maximal virtual size of a process, in kilobytes.
-w
swapuse
- The maximum amount of swap space reserved or used for this user ID, in
kilobytes.
umask
[-S
]
[mask
]
- Set the file creation mask (see
umask(2))
to the octal or symbolic (see
chmod(1))
value specified by mask. If the argument
is omitted, the current mask value is printed. If the
-S
option is specified, the output is
symbolic, otherwise the output is octal.
unalias
[-a
]
[name ...
]
- The specified alias names are removed. If
-a
is specified, all aliases are
removed.
unset
[-fv
]
name ...
- The specified variables or functions are unset and unexported. If the
-v
option is specified or no options
are given, the name arguments are treated
as variable names. If the -f
option is
specified, the name arguments are treated
as function names.
wait
[job ...
]
- Wait for each specified job to complete
and return the exit status of the last process in the last specified
job. If any
job specified is unknown to the shell, it
is treated as if it were a known job that exited with exit status 127. If
no operands are given, wait for all jobs to complete and return an exit
status of zero.
When
sh
is being used interactively from a
terminal, the current command and the command history (see
fc
in
Built-in Commands) can
be edited using
vi
-mode command line
editing. This mode uses commands similar to a subset of those described in the
vi(1)
man page. The command “
set -o vi
” (or
“
set -V
”) enables
vi
-mode editing and places
sh
into
vi
insert mode. With
vi
-mode enabled,
sh
can be switched between insert mode and
command mode by typing ⟨ESC⟩. Hitting ⟨return⟩
while in command mode will pass the line to the shell.
Similarly, the “
set -o emacs
” (or
“
set -E
”) command can be used to enable
a subset of
emacs
-style command line
editing features.
The following environment variables affect the execution of
sh
:
ENV
- Initialization file for interactive shells.
LANG
,
LC_*
- Locale settings. These are inherited by children of the shell, and is used
in a limited manner by the shell itself.
OLDPWD
- The previous current directory. This is used and updated by
cd
.
PWD
- An absolute pathname for the current directory, possibly containing
symbolic links. This is used and updated by the shell.
TERM
- The default terminal setting for the shell. This is inherited by children
of the shell, and is used in the history editing modes.
Additionally, environment variables are turned into shell variables at startup,
which may affect the shell as described under
Special Variables.
- ~/.profile
- User's login profile.
- /etc/profile
- System login profile.
- /etc/shells
- Shell database.
- /etc/suid_profile
- Privileged shell profile.
Errors that are detected by the shell, such as a syntax error, will cause the
shell to exit with a non-zero exit status. If the shell is not an interactive
shell, the execution of the shell file will be aborted. Otherwise the shell
will return the exit status of the last command executed, or if the
exit
builtin is used with a numeric
argument, it will return the argument.
builtin(1),
chsh(1),
echo(1),
ed(1),
emacs(1),
kill(1),
printf(1),
pwd(1),
test(1),
vi(1),
execve(2),
getrlimit(2),
umask(2),
wctype(3),
editrc(5),
shells(5)
A
sh
command, the Thompson shell, appeared in
Version 1 AT&T UNIX. It was superseded in
Version 7 AT&T UNIX by the Bourne shell,
which inherited the name
sh
.
This version of
sh
was rewritten in 1989
under the
BSD license after the Bourne shell from
AT&T System V Release 4 UNIX.
This version of
sh
was originally written by
Kenneth Almquist.
The
sh
utility does not recognize multibyte
characters other than UTF-8. Splitting using
IFS does not recognize multibyte
characters.